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Transcript of Organic photovoltaics using novel pentacene derivatives 1430/fulltext.pdf Organic Photovoltaics...

  • 1

    Organic Photovoltaics using Novel

    Pentacene Derivatives

    A dissertation presented

    by

    Anup Kumar Singh

    to

    the Department of Electrical and Computer Engineering

    in partial fulfillment of the requirements

    for the degree of

    Doctor of Philosophy

    in the field of

    Electrical Engineering

    Northeastern University

    Boston, Massachusetts

    December, 2013

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    Abstract

    Organic photovoltaic (OPV) technology has attracted scientific and economic interest over the

    last decade due to the possibility of low-cost manufacturing and the rapid increase in OPV power

    conversion efficiency. The future potential of these devices and the increasing demand for green

    energy have motivated government and private organizations and researchers.

    In OPV cells, excitons are generated in the organic semiconductor materials after absorption of

    photons. These excitons diffuse towards the electrode-semiconductor interface or donor-acceptor

    (p-type and n-type) interface, dissociate, and move by drift and diffusion processes toward the

    respective electrodes to generate electricity.

    Organic semiconductors suffer from low mobility resulting low short-circuit current and hence

    low power conversion efficiency (PCE). It is therefore critical to investigate an optimal value for

    which cells can perform the best. We use a new model to determine the optimum mobility for

    which the PCE is maximum. It is observed that PCE does not improve above this optimum

    mobility value of about 10 -4

    cm 2 /V-s. Donor and acceptor thicknesses are also optimized using

    the simulation program to get the best efficiency. We also investigate the effect of exciton

    diffusion length of organic semiconductor on power conversion efficiency, and simulations

    suggest that PCE almost saturates above certain value of exciton diffusion length.

    One of the most promising organic semiconductors is pentacene, which acts mainly as a donor

    molecule. We explore the efficacy of various new, very stable pentacene derivatives synthesized

    by Prof. Glen Miller’s group at University of New Hampshire, Durham. These pentacenes are

    water soluble pentacene (WSP); 5,6,7-trithiapentacene-13-one (TTPO), and Bis-C60 adduct of

    6,13-(decylthio)pentacene (BC60DTP). We, in this thesis, present the OPV cell designs (bilayer

    or planar and bulk heterojunction type), fabrication steps, electrical results and discuss them to

    correlate the experimental findings with simulation data.

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    Acknowledgements

    My first debt of gratitude must go to my thesis advisor, Prof. Nicol McGruer. I appreciate all his

    contributions of perennial guidance, valuable times, ideas and funding to complete my Ph.D.

    study. He motivated me to be confident in taking up any new difficult challenges and I came up

    with success. I sincerely thank him for many insightful discussions of semiconductors,

    photovoltaic theories and microfabrication, which helped me a lot during my Ph.D. pursuit.

    Special thanks to my committee thesis readers, Prof. Glen Miller and Prof. Matteo Rinaldi for

    their valuable suggestions during my thesis and proposal defense. My heartfelt thanks go to Prof.

    Glen Miller for providing me with various newly synthesized organic semiconducting materials

    to carry on my photovoltaic research. We had several useful meetings and insightful discussions

    with Prof. Glen and his research group members Chandrani, John, Jeremy, and Yushu.

    I am grateful to Prof. Ahmed Busnaina, the director of the Center for High-rate

    Nanomanufacturing (CHN), who funded me as a Research Assistant. While working at the

    microfabrication facility of the Kostas Nanoscale Research Center, the lab manager Scott

    McNamara, who helped me with training on various equipment, deserves my sincere thanks.

    I sincerely thank Dr. Siva Subramanian, from whom I learned many things in device physics and

    microfabrication. He has been a great colleague, mentor and supportive. My friends, and

    colleagues in the lab, Erfan, Saba, Praba, Suchit, Mithun, Kapil, Ritesh, Peter, Hunchul, and

    especially Kamal with whom I had many technical as well as informal discussions, were sources

    of laughter, joy and support.

    I would be thankless if I don’t mention the indirect contribution of my villagers in India who had

    huge expectations from me since my childhood. This encouraged me to work hard.

    The love and support given by my parents, my brother and sisters, and my wife were incessant

    and truly inspiring, for which my mere expression of gratitude does not suffice. Our newly born

    angel Arohi filled me with joy and energy toward the end of my Ph.D. Thank you.

    Anup Kumar Singh

    Northeastern University

    December 2013

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    I dedicate this thesis to

    my parents, my brother and sisters,

    my wife, and our little angel.

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    Contents

    ABSTRACT .................................................................................................................................... 2

    ACKNOWLEDGEMENTS .......................................................................................................... 3

    TABLE OF FIGURES ................................................................................................................... 9

    1. MOTIVATION AND OUTLINE ....................................................................................... 15

    1.1 THE BACKGROUND ........................................................................................................ 15

    1.1.1 Limited Fossil Fuels and Environmental Impact ................................................... 15

    1.1.2 Renewable Sources of Energy ............................................................................... 16

    1.1.2.1 Wind Power .................................................................................................................... 16

    1.1.2.2 Hydro Power ................................................................................................................... 16

    1.1.2.3 Solar Energy ................................................................................................................... 17

    1.1.3 Photovoltaic Technology and Development .......................................................... 17

    1.1.4 Photovoltaic Manufacturing: Present and Future ................................................... 19

    1.1.5 Solar Cell Types ..................................................................................................... 20

    1.1.5.1 Solar Cells of Crystalline Silicon .................................................................................... 20

     Monocrystalline Silicon (c-Si): ............................................................................................... 21

     Polycrystalline Silicon (poly-Si): ............................................................................................ 21

    1.1.5.2 Thin Film Solar Cells ...................................................................................................... 21

     Amorphous Silicon (a-Si): ....................................................................................................... 21

     Gallium Arsenide (GaAs) ........................................................................................................ 22

     Cadmium Telluride (CdTe) ..................................................................................................... 22

    1.1.5.3 Dye Sensitized Solar Cell (DSSC) .................................................................................. 22

    1.1.5.4 Quantum Dot Solar Cell (QDSC) ................................................................................... 23

    1.1.5.5 Organic Solar Cell (OSC) ............................................................................................... 23

    1.2 MOTIVATION OF THIS THESIS ........................................................................................ 24

    1.3 OUTLINE ......................................................................................................................... 26

    2. ORGANIC SOLAR CELL – GENERAL .......................................................................... 27

    2.1 INTRODUCTION ............................................................................................................... 27

    2.2 PRINCIPLE OF OPERATION .............................................................................................. 28

    (a) Photon Absorption .............................................................................................................. 29

    (b) Exciton Generation and Diffusion ....................................................................................... 29

    (c) Exciton Dissociation ........................................................................................................... 30

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    (d) Charge Transport ................................................................................................................. 30